Polycarbonate, epoxy resin and so forth have frequently been used as the substrate materials of conventional optical disks since these substrate materials are required to have low contents of extraneous materials and impurities, high permeability and low double refractive index to enable stable reading and writing, a low moisture absorption rate and superior heat resistance to prevent deformation of the optical disk, as well as high fluidity and superior mold release to facilitate molding processing (Japanese Unexamined Patent Application, First Publication No. 05-258349).
However, since bisphenol A is used as monomer for the aforementioned polycarbonate and epoxy resin substrate materials, unreacted bisphenol A remained even after polymerization. Due to the growing interest in environmental issues in recent years, materials containing bisphenol A have tended to be shunned, and studies have been conducted on substrate materials that do not contain bisphenol A.
The use of glass has been considered for use as a substrate material since it does not contain bisphenol A and satisfactory optical characteristics in the form of high transmittance. However, glass substrates have problems in terms of strength in that they are unable to accommodate the pressure and stress during production and use due to limitations on disk thickness.
Another problem with conventional optical disks is that, although silk screen printing has mainly been used in the case of printing characters or images on the surface, there is the problem of the difficulty in obtaining highly detailed images with silk screen printing.
In addition, in the case of silk screen printing, in order to print serial numbers or other different characters and images for each disk, it is necessary to change the form plate each time, thereby resulting in the problem of being unable to in fact impart variable information by printing to conventional optical disks.
Accordingly, an object of the present invention is to provide an optical disk, and manufacturing method of the same, which has performance equal to that of conventional optical disks and has a minimal effect on the environment during disposal.
In addition, another object of the present invention is to provide an optical disk on which highly detailed images are printed, and manufacturing method of an optical disk that enables highly detailed images to be printed inexpensively and impart variable information by printing.